The present invention is related to wireless networks, and in particular to methods and apparatuses for transmitting using multiple antennas without requiring the receiver of the transmission to have multiple antennas.
Wireless networks, e.g., local area wireless networks (WLANs) conforming to the IEEE 802.11 standard have become common. It is known that the performance of a link in such a WLAN is significantly degraded in the presence of multipath, as in an office setting where there is no line-of-sight from the client to the access point. Some variants of the IEEE 802.11 standard use orthogonal frequency division multiplexing (OFDM), which is known to perform better than many alternatives in the presence of multipath.
WLANs often are used in an infrastructure wherein one wireless station of the network, called an access point, acts as a base station for a set of client stations. One mechanism for improving communication is to use multiple antennas at the access point and possibly at the client stations.
It is known, for example, to use antenna selection diversity at the access point wherein one of a plurality of receive antennas is selected according to a selection criterion, typically signal strength at the two receivers as measured by the received signal strength indication (RSSI) signal at the radio receiver. U.S. patent application Ser. No. 10/698,588 to Lyons et al. filed Oct. 31, 2003 titled ERROR VECTOR MAGNITUDE SELECTION DIVERSITY METRIC FOR OFDM, published as US 20050208897A, and now U.S. Pat. No. 6,922,549, introduced an alternate measure for antenna selection in an OFDM receiver based on an error vector magnitude (EVM) measure obtained at the receiver and measured from a preamble part of a packet as used in WLANs and received at the receiver.
It also is known to use beamforming at the access point, e.g., to use multiple radio receivers, one per receive antenna, and then combine the received signals from each antenna according to a combining method.
These methods significantly improve reception at the access point. Of course one can similarly improve reception at the client for transmissions by the access point by including multiple antennas at the client station. It would be beneficial, however, for the client to remain single antenna to maintain lower cost.
One known method of maintaining single antenna clients while having symmetry in the quality of reception at the access point (the uplink direction) and the quality of reception at the client (the downlink direction) is to include receive diversity at the access point for uplink improvement and transmit at higher power on the downlink. The higher transmit power, however, increases the likelihood of co-channel interference in an environment that includes several access points.
Thus there is a need in the art for methods of transmitting using multiple transmit antennas.
One known multiple antenna transmit solution includes changing which transmit antenna is used when a packet fails to be received at the client. This technique effectively involves transmit selection diversity at the media access control (MAC) level.
Thus there is still a need in the art for methods and apparatuses of transmitting using multiple transmit antennas.
There further is a need in the art for methods and apparatuses of transmitting using multiple transmit antennas that do not require the receiver, e.g., a single antenna client, to exchange knowledge related to calibration.